A vehicle such as an automobile is equipped with a stabilizer (a stabilizer bar or an anti-roll bar) for reducing the body roll of the vehicle which is caused by upward and downward shifts of the wheels. Generally, the stabilizer is made from a substantially U-shaped bar body, and includes: a torsion section extending in the vehicle width direction; and a pair of left and right arm sections bent in the vehicle front-rear direction. In the vehicle, the stabilizer is supported and suspended between the left and right suspensions of the wheels, with the distal ends of the arm sections respectively linked to the suspensions, and with the torsion section inserted through the bushes fixed to the vehicle body.
While the vehicle is cornering, or while the vehicle is running over a bump on the road surface, upward and downward motions of the left and right wheels make the strokes of the left and right suspensions differ from each other. In this case, load (displacement) caused by the difference in stroke between the suspensions is inputted into the arm sections of the stabilizer; resultant load (displacement difference) from the arm sections twists the torsion section; and the torsion section produces elastic force for recovering from the twist deformation. Using the elastic force for recovering from the twist deformation, the stabilizer reduces the difference in vertical displacement between the left and right wheels, increases the roll stiffness of the vehicle body, and accordingly reduces the body roll of the vehicle.
There are two structurally-different types of stabilizers: a solid stabilizer having a solid structure, and a hollow stabilizer having a hollow structure. The features of the solid stabilizer include excellence in mechanical strength and reduction in manufacturing costs. In contrast to this, the hollow stabilizer is in the form suitable to reduce the weight of the vehicle, although having difficulty in achieving mechanical strength in comparison with the solid stabilizer. In general, electroseamed steel pipe, seamless steep pipe, forge welded steel pipe and the like are used as materials for the hollow stabilizer. Among these steel pipes, electroseamed steel pipe is used most as the material for the hollow stabilizer because of its low manufacturing costs and excellent mass productivity.
Conventional materials generally used for stabilizers are carbon steels such as S48C (JIS), and spring steels such as SUP9 (JIS) and SUP9A (JIS) having excellent mechanical strength such as tensile strength, and excellent fatigue resistance. Hollow stabilizers are often produced by bending steel pipe made of spring steel into a product shape, followed by heat treatment. As the bending process, cold bending using NC benders, hot bending using a bending die for a whole stabilizer, or the like is performed depending on the thickness and diameter of steel pipe.
As the heat treatment, quenching and tempering are performed. As a method of quenching, oil quenching is used most often. Heat-treated pipe is made into a product usually through a surface treatment process such as shot peening, and a finishing process such as coating.
Inventions publicly known by documents related to this application are disclosed in Patent Literatures 1, 2 listed below.
For example, Patent Literature 1 discloses that electric-resistance welded steel pipe for hollow stabilizers where a percent ratio of plate thickness t to outer diameter D is t/D≥20% is achieved by using electric-resistance welded steel pipe which is obtained by diameter-reduction rolling after electric-resistance welding.
Patent Literature 2 discloses, as a technique for securing durability for stabilizers, a hollow stabilizer manufacturing method which includes: subjecting electroseamed pipe to diameter reduction within a hot or warm temperature range to make a ratio of plate thickness to outer diameter fall within 18 to 35%; and forming the diameter-reduced electroseamed pipe into a stabilizer shape, followed by heat-treatment, shot-peening and coating.